1. Field of Invention
The present invention relates to a chemical-mechanical polishing (CMP) station. More particularly, the present invention relates to a chemical-mechanical polishing station that has a polishing table capable of improving the uniformity of a polished surface of a wafer.
2. Description of Related Art
As the level of integration of semiconductor devices continues to increase, designs having two or more metallic layers for interconnecting additional MOS transistors brought about by device miniaturization are quite common. Normally, the metal interconnect structures are isolated by inter-layer dielectrics (ILD) or inter-metal dielectrics (IMD). When devices are miniaturized, the corresponding design rules for producing the ILD or the IMD layer are also more restrictive. For example, the degree of planarization necessary on the surface of an ILD or an IMD layer is correspondingly higher.
In order to simplify the fabrication of multiple-level metallic interconnects and to increase accuracy of conductive line pattern transfer, planarization of a wafer to remove surface undulation is very important. The degree to which a wafer is planarized is a critical factor in the precise alignment of a photomask. If the wafer is not properly planarized a computerized system will be unable to align the photomask correctly, resulting in a considerable increase in the probability of processing errors.
In the manufacturing of semiconductor devices, surface planarization is an important step in preparing a wafer for high-resolution photolithographic processing. Only a smooth surface with little height variation can prevent diffraction of light from a light source when a pattern is transferred. In general, planarization techniques include a spin-on-glass (SOG) method and a chemical-mechanical polishing method.
However, in the sub-half-micron device era, the spin-on-glass method of planarization is incapable of providing the degree of planarity required by photolithographic operation. Consequently, chemical-mechanical polishing has become the chief method of providing global planarization up to the level of planarity required for fabricatiing devices in very-large scale integration (VLSI) or even ultra-large scale integration (ULSI) circuits.
Chemical-mechanical polishing (CMP) is a technique of planarizing wafer surfaces using mechanical polishing together with a suitable chemical reagent. Generally, the chemical reagent used in the polishing operation is called a slurry or a polishing solution. The slurry for carrying out a CMP operation is usually colloidal silica or a solution of dispersed alumina in an alkaline media such as potassium hydroxide (KOH) or ammonium hydroxide (NH.sub.4 OH). The hard alumina or silica particles within the slurry have sizes ranging from 0.1 to 2.0 .mu.m. Principally, the surface of a wafer is polished using these hard particles suspended within the slurry. In addition, the slurry is also filtered before it is transported to the polishing pad of the CMP station. Filtering the slurry helps to remove any impurities that may cause unnecessary scratching damage to the wafer surface.
FIGS. 1A and 1B are top and side views of a conventional chemical-mechanical polishing station. As shown in FIGS.1A and 1B, the station includes a polishing table 10, a wafer holder 11 for grasping a wafer 12, a polishing pad 13 over the polishing table 10, a tube 14 for carrying slurry 15 to the polishing pad 13, a liquid pump 16 for pumping slurry 15 to the tube 14, and a conditioner 17 for dressing the surface of the polishing pad 13. When the chemical-mechanical polishing station is carrying out a polishing action, the polishing table 10 and the wafer holder 11 each rotate independently in a pre-defined direction, for example, directions 18a and 18b, respectively.
The wafer holder 11, while gripping the backside 19 of the wafer 12, presses the front face 20 of the wafer 12 onto the polishing pad 13. The liquid pump 16 also works continuously to pump slurry 15 to the polishing pad 13 through the tube 14. The polishing action in a chemical-mechanical polishing operation relies on chemical reagents and the abrasive particles suspended in the slurry. The reagents react chemically with molecules on the front surface 20 of the wafer 12 to form an easy-grind layer, while the abrasive particles of the slurry 15 help to remove pointed peaks within the easy-grind layer. By continuous chemical reaction and repeated mechanical abrasion, a highly polished and planar surface is ultimately formed on the wafer surface.
The polishing table 10 rotates at a definite angular speed in the direction 18a shown in FIGS. 1A and 1B. However, the wafer 12 on the polishing pad 13 is polished a tangential speed that depends on the radial distance from the center of the circular table 10. This is because the actual polishing speed or tangential speed is the product of the angular speed and the radius. Since tangential speed increases proportionately from the center towards the perimeter of the polishing table, the amount of polishing received by the front surface 20 of the wafer 12 varies according to its location on the polishing pad 13. Consequently, the degree of planarity after the polishing operation may vary considerably due to differences in tangential polishing speed.
In light of the foregoing, there is a need to provide an improved polishing table design for a chemical-mechlanical polishing station capable of producing a highly planar wafer surface.